The present invention relates to a process and an apparatus for transfering electrostatic latent images in electrophotography, more particularly for transferring electrostatic latent images formed on a photosensitive plate, one dielectric element, to copy paper, another dielectric element, free of damage to the images to obtain clear and sharp copies.
For the transfer of electrostatic latent image, a photosensitive plate is generally used which comprises an electrically conductive base sheet in the form of a drum or belt and a dielectric layer formed on the base sheet and made up of a single photosensitive layer of amorphous selenium, zinc oxide, organic semi-conductor, or the like, or of a composite photosensitive layer composed of single layers of such materials. The process of transferring an electrostatic latent image includes the step of forming a latent image on the surface of the photosensitive plate by exposing the surface to light reflected from or passing through an original after the surface has been charged by corona discharge or the like, and the step of bringing copy paper into intimate contact with the photosensitive plate by an electrically conductive roller made of conductive rubber, metal or the like, to transfer the latent image to the copy paper, the copy paper having on its front face a dielectric layer of a high electric resistance (at least 10.sup.12 ohms) and on its rear face an electrically conductive layer of a high resistance (10.sup.5 to 10.sup.10 ohms).
If the copy paper is brought into intimate contact with the photosensitive plate by one grounded roller during the transfer process described, a larger electric energy gap will be produced between the photosensitive plate bearing the electrostatic latent image at a high potential (at least 1000 volts) and the copy paper at nearly zero potential when there exists a relatively large gap therebetween before they come into intimate contact with each other, with the result that abnormal discharge will take place, partially damaging the latent image. Thus it is very difficult to transfer the latent image to the copy paper free of damage, and objections will result such an non-developed spots in the developed image on the copy.
To eliminate such drawback, processes have been proposed as disclosed for example in U.S. Pat. No. 3,147,679 by R. M. Schaffert and Publication of Japanese patent application No. 19757/1967 by Shaffert et al.
The process of U.S. Pat. No. 3,147,679 is schematically shown in FIG. 1 in which an electrostatic latent image is formed by well known means on a photosensitive plate 4 on the surface of a grounded drum, and copy paper 3 is brought into intimate contact with the latent image bearing surface by a first roller 1 provided with a voltage of the same polarity as the charge on the image bearing surface. Subsequently, the latent image is transferred by a second roller 2 applied by a potential source 7 with a high voltage of polarity opposite to that of the charge of the latent image. A charger 6 previously places a potential of polarity opposite to that of the latent image on the dielectric layer of the copy paper 3, so that in order to prevent abnormal dishcharge between the photosensitive plate 4 and the copy paper 3 immediately before the aforementioned intimate contact is achieved by the first roller 1, there is provided a variable potential source 5 capable of supplying from zero to a relatively high potential of several thousand volts to the first roller 1. The variable potential source 5 further applies to a separating roller 8 a voltage of the same polarity as the roller 1 to prevent damage due to discharge when the copy paper 3 is separated. Thus the disclosed process requires the variable potential source 5 which is very complex and large. Furthermore, since transfer is effected through field emission, the gap between the photosensitive plate 4 and the copy paper 3 must be kept very small, for instance, not greater than 10 microns. The coarseness of the surface of the copy paper 3 therefore exerts a serious influence on the transfer effect, and the copy paper must be highly smooth surfaced, hence limitation on the quality of the copy paper. The application of a high voltage on the second roller further requires sufficient care to assure safety.
The process of Publication of Japanese Patent Application No. 19757/1967, an improvement over the process illustrated in FIG. 1, is schematically shown in FIG. 2. According to this process, there is provided a first roller 1 for bringing copy paper 3 into contact with a photosensitive plate 4' with an electrostatic latent image formed on its dielectric surface by known means, before the paper 3 reaches a grounded electrically conductive roller (second roller) 2'. A variable potential source 5 applies to the roller 1 a potential of the same polarity as the surface charge of the photosensitive plate 4' to prevent abnormal discharge before the paper is brought into intimate contact with the plate and to thereby eliminate damage to the transferred latent image. A drive roller 9 for the photosensitive plate 4' is grounded.
This process has overcome the drawback of the process of FIG. 1 that a high voltage must be applied to the second roller, but it is still necessary to use the complex and large variable potential source 5 for the first roller 1 as in the case of FIG. 1. Instead of applying a high voltage to the second roler 2', the second roller 2' is merely grounded and is subjected to a high pressure, for example of 21 kg/cm.sup.2 (about 300 lb/in.sup.2) to control the gap between the photosensitive plate 4' and the paper 3 to a very small width (for example not greater than 3 microns) such that the air will be expelled from the gap. Accordingly, the second roller must be provided with pressure means 10 which is complex, with further consideration given to render the operation means strong enough to withstand the high pressure. It is also necessary to use very smooth surfaced transfer paper.
The process of FIG. 1 involves latent image transfer which utilized air breakdown discharge across a small gap of not greater than 10 microns between the photosensitive plate and copy paper with the application of high voltage, in combination with latent image transfer by field emission at a smaller gap, whereas the process of FIG. 2 is a latent image transfer process by field emission at a very small gap (for example not greater than 3 microns). Either process requires an extremely high degree of smoothness for the contact surfaces of both copy paper and photosensitive plate as well as a complex and large mechanism which is resistant to pressure and voltage.